1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus which projects and transfers a circuit pattern drawn on a reticle (mask) onto, for example, a wafer via a projection optical system has conventionally been employed to fabricate a semiconductor device using photolithography.
Along with the micropatterning of semiconductor devices, the projection exposure apparatus is required to transfer a reticle pattern onto a wafer by exposure with a higher resolving power. A minimum line width (resolution) that the projection exposure apparatus can transfer is proportional to the wavelength of exposure light and is inversely proportional to the numerical aperture (NA) of the projection optical system. In view of this, the wavelength of the exposure light is shortening and the NA of the projection optical system is increasing.
An exposure light source has currently shifted from a superhigh pressure mercury lamp (i-line (wavelength: about 365 nm)) to a KrF excimer laser (wavelength: about 248 nm) and an ArF excimer laser (wavelength: about 193 nm), and the practical application of even an F2 laser (wavelength: about 157 nm) is in progress. Moreover, the adoption of EUV (Extreme Ultra Violet) light with a wavelength of about 10 nm to 15 nm is expected.
There has also been proposed immersion exposure that aims at increasing the NA of the projection optical system by filling at least part of the space between the projection optical system and the wafer with a liquid (e.g., a liquid with a refractive index higher than 1). The immersion exposure improves the resolution by increasing the NA of the projection optical system on the wafer side.
Along with such an improvement in resolution, the projection exposure apparatus is also required to improve the overlay accuracy, that is, the accuracy of overlaying several patterns on the wafer. In general, the overlay accuracy must be about ⅕ the resolution. Along with the micropatterning of semiconductor devices, it is increasingly becoming important to improve the overlay accuracy. To obtain a desired overlay accuracy, it is necessary to align the reticle and wafer with high accuracy. For this purpose, the projection exposure apparatus includes a plurality of alignment detection systems (i.e., position detection apparatuses).
Wafer alignment detection systems are roughly classified into two, that is, the off-axis detection system and the TTL-AA (Through the Lens Auto Alignment) detection system. The off-axis detection system detects an alignment mark on the wafer without using a projection optical system. The TTL-AA detection system detects an alignment mark on the wafer with the alignment wavelength of non-exposure light via a projection optical system.
In recent years, the semiconductor device production mode is shifting from low-variety, high-volume production to high-variety, low-volume production. Along with this trend, an alignment detection system which can minimize detection errors in wafer processes under various conditions (with regard to, e.g., the material, thickness, film thickness, and line width) is demanded. For example, when the alignment detection system includes a TIS (Tool Induced Shift), it generates detection errors even when it detects an alignment mark with a symmetrical stepped structure. Detection errors are generated due to aberrations (especially, coma aberration due to decentering) which cause the TIS and remain in the optical system of the alignment detection system, and the tilt (optical axis shift) of the optical axis of this optical system. To provide an alignment detection system which can minimize detection errors in wafer processes under various conditions, it is necessary to reduce coma aberration and an optical axis shift of the optical system of the alignment detection system.
Coma aberration of the alignment detection system is often reduced by moving an optical member of the alignment detection system (adjusting the optical center of gravity) so that an asymmetrical waveform obtained upon detecting an adjustment mark becomes symmetrical. See Japanese Patent Laid-Open No. 9-167738 for details of this technique. Then, an alignment mark (chromium pattern) included in the exposure apparatus is detected in each defocus state, and the alignment detection system is adjusted so that the detection position (defocus characteristic) of an image of the alignment mark falls within a predetermined range (specification).
However, the prior art adjusts the alignment detection system so that coma aberration and an optical axis shift of the optical system of the alignment detection system are canceled in total. In other words, the prior art does not reduce coma aberration and an optical axis shift of the optical system of the alignment detection system to zero.
A waveform obtained upon detecting the adjustment mark becomes asymmetrical not only due to the influence of coma aberration but also due to the influence of an optical axis shift. In some cases, even when the alignment detection system is adjusted so that the waveform symmetry falls within a predetermined range (specification), an asymmetrical waveform component due to an optical axis shift is merely canceled by the influence of coma aberration, so the coma aberration and optical axis shift, in fact, remain in the alignment detection system. When the alignment detection system is adjusted so that the defocus characteristic as an index of an optical axis shift of the wafer alignment detection system satisfies a specification while the coma aberration and optical axis shift remain in the alignment detection system, the waveform symmetry may deteriorate, resulting in detection errors.
The alignment detection system detects the alignment mark by selecting a wavelength range, in which the contrast of the detection waveform is highest, for each wafer process. If a certain wafer process cannot obtain a required contrast in wavelength ranges provided to the alignment detection system, a new wavelength range in which a required contrast is obtained is sometimes additionally set for it. When a new wavelength range is additionally set for the alignment detection system, it is necessary to adjust the alignment detection system so that the defocus characteristic in the new wavelength range satisfies a specification. However, even in this case, when the alignment detection system is adjusted so that the defocus characteristic in the new wavelength range satisfies the specification, the waveform symmetry may deteriorate, resulting in detection errors.